Those familiar with the current state of the telephone art are aware that there have been numerous and varied proposals for providing directory assistance through telephonic access to centralized data banks such as metropolitan telephone number directories. These systems generally comprise arrangements for accessing the data through a standard "Touch Tone" (DTMF) key pad and have encompassed a large variety of schemes for eliminating the ambiguities caused by the multiple alphanumeric characters signified by each key.
U.S. Pat. No. 4,427,848 to Tsakanikas describes a system wherein alphabetic characters are transmitted by depressing a designated key a number of times equal to the relative position of the inscription of the character on the key, followed by the depression of the key on which the character is inscribed.
Systems which are known in the simultaneous depression of a plurality of keys are used to produce a signal having frequency characteristics which may be discriminated from the pairs of frequencies generated in response to the depression of a single key.
U.S. Pat. No. 3,618,038 to Stein utilized what is known as the delayed depression translation technique, wherein depression of keys having different durations are discriminated.
These systems in general are slow, tedious, involve a coding scheme which is difficult for a user to master, and are more or less impractical for general acceptance or use. This problem has been recognized and produced various other proposals for surmounting certain aspects of the difficulties.
U.S. Pat. No. 4,677,659 to Dargan teaches a DTMF system which is usable without employing an encoding scheme for each alphabetic letter. According to this patent the principle of Boolean exclusion is used in a specific fashion to discriminate among data base entries indexed by numerals matching the letters designated on a standard DTMF telephone key pad. At the same time this system discriminates among separate entries with matching index numbers by the content of their informational fields. This is accomplished by multiple strings of key pad entries defining progressively larger percentages of the content of the informational field.
U.S. Pat. No. 4,817,129 to Riskin describes DTMF access to a data base by utilizing a system of filettes which comprise abbreviated versions of the data base.
U.S. Pat. No. 4,782,509 to Shepard is directed to an automatic directory assistance system which is usable over a large geographic area involving a plurality of local exchanges. The party seeking assistance enters all of the request data through the DTMF keys. Assistance is provided in the form of a voice response identifying the name of the sought subscriber and the telephone number. It is a feature of this patent that the DTMF signals are transformed to digital form for processing. The located alphanumeric information including name and telephone number is then sent back through the digital transmission system to the request processor. The alphanumeric code data is used at that processor to control a voice response unit.
U.S. Pat. No. 3,928,724 to Byram is directed to a voice actuated automatic telephone directory assistance system wherein the user verbally spells the name of the sought party letter by letter.
U.S. Pat. No. 4,593,157 to Usdan is directed to automatic directory assistance responsive to speech recognition circuitry. The response to the inquiry is provided in the form of a display on the inquiring instrument. An automatic dial out feature is included.
U.S. Pat. No. 4,608,460 to Carter et al discloses a comprehensive automatic directory assistance apparatus that is subscriber accessed through a conventional telephone exchange and which receives DTMF input signals and responds via synthesized voice. At the option of the caller connection may be automatically made to the identified subscriber. The methodology comprises obtaining from the caller a limited number of key depressions to spell the last name of the sought party even though this may not completely spell that name. On the instances where the data bank contains multiple matches to the limited information provided by the caller the caller is presented with a selection of all such matches vocally for selection by depressing a predesignated DTMF key. Optionally the caller can elect to have the selected number dialed automatically by the system.
While all of the foregoing systems are theoretically feasible for providing an automatic electronic telephone directory for public access they individually suffer varying disadvantages, such as for example, substantial cost, special equipment, complex establishment and maintenance procedures and modification of existing networks.